1. Field of the Invention
The present invention relates generally to the synthesis of metal powders and films, and more specifically, to the synthesis of nanostructured metal powders and coatings using a millimeter wave gyrotron beam.
2. Description of the Related Art
Metallic powders have been prepared by physical vapor deposition, mechanical blending and mixing, and by chemical routes. Vapor methods are not cost effective and make only small amounts of material. The mechanical blending route often introduces impurities into the final product. Fluidized beds have also been used to coat powders with metals, however, as in vapor methods, the initial equipment is expensive and it is difficult to evenly coat powders and to handle powders of different sizes.
Metallic coatings have been prepared using electroplating and electroless plating. Electroless plating requires that the substrate first is pretreated before plating and the substrate must also be an insulator. Using the polyol method, there does not need to be any chemical pretreatment of the surface and the substrates may be either conductive or insulators.
Nanostructured powders and films (with particle diameters of about 1-100 nm) have many potential electronic, magnetic, and structural applications such as catalysis, electromagnetic shielding, ferrofluids, magnetic recording, sensors, biomedical, electronics and advanced-engineered materials.
Among the various preparative techniques, chemical routes offer the advantages of molecular or atomic level control and efficient scale-up for processing and production. Others in the art have prepared micron and submicron-size metallic powders of Co, Cu, Ni, Pb and Ag using the polyol method. These particles consisted of single elements. Depending on the type of metallic precursors used in the reaction, additional reducing and nucleating agents were often used. The presence of the additional nucleating and reducing agents during the reaction may result in undesirable and trapped impurities, particularly non-metallic impurities.
These prior procedures have been unable to obtain nanostructured powders having a mean size of 1-25 nm diameter. These prior procedures have not been useful in producing nanostructured powders of metal composites or alloys or metal films.
Accordingly, it is an object of this invention to form nanostructured metal products.
It is a further object of this invention to form nanostructured metallic powders and coatings.
It is a further object of this invention to form nanostructured powders and coatings of metal alloys and metal/ceramic composites.
It is a further object of the present invention to form nanostructured powders and coatings of metals without the need to use a nucleating agent.
It is a further object of the present invention to form nanostructured powders and coatings of metals using a millimeter wave beam.
In this invention, metal salts of the material of interest are suspended or dissolved in a glycol. When substrates are to be coated, the substrate is placed in the metal/glycol mixture. Suspension may be accomplished by proper choice of glycol, powder size, combined with ultrasonification, mechanical agitation or stirring, or utilizing convection due to bubbling. The mixture is heated to reflux and the reduction to the metallic state occurs. As the reduction proceeds, the metal nuclei combine to form nanosized particles, which may remain in the colloidal state, or be further processed to form larger particles which will precipitate out of solution. The precipitated powders may be removed from solution by filtration or centrifugation. The colloidal metallic particles can be concentrated by centrifugation. The concentration of the metal precursor salt, time and temperature of reflux can control coating thickness and particle size. As well as the time required, temperature achieved and rate of temperature change is controlled by the amount of powder in the beam, which can be changed rapidly. Due to the volumetric heating nature of the millimeter wave beam, the heating of the solution can be rapidly turned off and does not rely upon the thermal inertia of conventional heating sources. This, coupled with the small volume sizes, allows for the almost immediate cessation of processing. Precursor salts that can be used include: chlorides, acetates, acetylacetonates, oxides, acids, carbonyls, carbonates, hydrates, hydroxides, nitrates and the glycols used include: ethylene glycol, propylene glycol, tetraethylene glycol, ethoxyethanol, ethylene glycol monoethyl ether, and diethylene glycol diethyl ether.